This revised application requests two postdoctoral positions annually for MD or MD/PhD fellows to perform basic research in virology or microbial immunology at the University of Pennsylvania. The candidate pool will come primarily from the adult ID fellowship program; however, additional candidates will come from pediatric ID, other postdoctoral MD training programs at Penn, particularly those with mentors on the training grant, including neurology, pathology, hematology-oncology, and pulmonary medicine, and by advertising for candidates outside Penn. The quality of our recent trainees is impressive, and has improved dramatically in part because of the many changes that have occurred on Penn's campus, including remarkable expansion biomedical research facilities since 1990, recruitment of superb investigators to fill new laboratories, exceptional growth in number and quality of ID and basic science faculty providing outstanding opportunities for clinical and research training, expansion of the number of combined degree (MD/PhD) physicians recruited into the Internal Medicine Residency Program who later become superb candidates for fellowship training, and an excellent environment fostered by close interactions between clinical and basic science faculty. Candidates will interview with the Executive Committee and other faculty, and those selected will enter the training grant after completing their clinical training. The trainers are renowned in the fields of herpes virology, HIV/AIDS, and microbial immunology. Those trainees who do not hold a PhD or equivalent degree will enroll in two core graduate-level courses in virology, and will have the opportunity to take additional courses in molecular biology and related disciplines while training in outstanding laboratories. The environment at Penn is superb for research at all levels, student, postdoctoral and faculty. The need for training physician-scientists has never been greater. The ID training program presented in this application provides an opportunity to train the next generation of leaders in academic Infectious Diseases.